Beam commissioning of STF accelerator at KEK

A compact high-brightness X-ray source based on inverse Compton scattering has been developed on the Superconducting Accelerator Test Facility (STF) at KEK. The beam line has an L-band normal conducting photo cathode RF gun, one Capture cryomodule with two 1.3 GHz 9-cell 1-m long superconducting cavities, a laser super optical cavity, and a beam dump. This is called quantum beam (QB) accelerator supported by MEXT in Japan. From accelerator, quasi-monochromatic X-ray via inverse Compton scattering is obtained. Our goal in the superconducting RF accelerator system is to establish the stable operation of a 20 MV/m accelerator field for a 1 ms-long pulse train, then to obtain 10,000 nC/pulse. We are attempting to demonstrate a high-brightness X-ray generation with target value of 1.3 x 1010 photons/sec 1% band width and with a 10-20 micron head-on collision. The beam commissioning for the photo injector was started from end of Feb 2012. About 7 mA flat beam with 1 ms pulse width was generated at Mar 2012. A cooldown of the capture cryomodule was started from middle of Feb 2012 for the rf process of the superconducting cavities. The gate valves of the capture module were opened to accelerate the electron beam to 40 MeV and to transport to the beam dump at middle of April 2012, then the commissioning of the beam tuning was started. The 1 ms beam with 3.2-6.5 mA peak current and 40 MeV was achieved at middle of July. The normalized emittance was achieved less than 1.5 mm mrad at the mirror collision point, and the 36 micron beam was also achieved at collision point without a beam loss. And an inspection of facility for the radiation safety was passed at middle of July. The beam operation will restart at Sep 2012, and the X-ray will be generated by the super optical cavity. In this report, the commissioning of the photo injector and the beam tuning will be reported. (author)